The present disclosure relates to sensors, and in particular to techniques for biasing of sensors.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Sensors are used for many purposes. For example, a temperature sensor may be incorporated on an IC chip (on-chip) for temperature measurement. A remote sensor may be provided off-chip and wired to a sense amplifier that is on-chip. An example of a emote sensor is in a magnetic sensor that is provided on read-write head of a magnetic disk drive device.
Typically, the output voltage of the sensor represents the property being measured. For example, a temperature sensor will have an output voltage that varies with temperature. A magnetic sensor will have an output voltage that varies with the strength of the magnetic field to which the sensor is exposed. Proper operation of a sensor typically requires biasing the sensor with a voltage so that the voltage swings of the sensor's output voltage vary linearly over the expected operating range of, for example, the temperature or magnetic field strength.
Sensors come in a variety of technologies, including passive resistive devices and active resistive devices such a MR (magneto resistive) devices, GMR (giant magneto resistive) devices, and TMR (tunneling magneto resistive) devices and so on. Biasing a sensor typically involves setting up a biasing current to flow through the sensor.
A current source can be employed to source enough current to set up a proper biasing voltage across the sensor that lies within the linear voltage region of the sensor. Current sources, however, tend to be noisy and current accuracy is difficult to achieve, especially if the sensor require high current levels to, set up a suitable bias voltage.
A resistor chain comprising a variable resistor connected in series with the sensor to a voltage source, Vsrc may be. The variable resistor can be adjusted so that enough current flows to set up the proper biasing voltage (Vbias) across the sensor. Given the desired bias voltage and knowing the resistance of the sensor (RS), it is a small matter of applying Ohm's law to determine a proper value (Rvar) of the variable resistor. For example, according to Ohm's law, Vsrc=I(Rvar+RS). The current that needs to flow through the sensor is computed as
            V      bias              R      S        .Therefore,
      V    src    =                    V        bias                    R        S              ×          (                        R          var                +                  R          S                    )      and
      R    var    =            R      S        ⁢                                        V            src                    -                      V            bias                                    V          bias                    .      This approach requires an accurate measurement of the sensor resistance. Moreover, variable resistor circuits for integrated circuits are process dependent, and so accuracy can be difficult to achieve.